As the title says, one of my clients has Invision forum (like this) and overnight all PM messages were gone, as well as some other features (like comments). Apparently, they are still listed in database, and they swear they didn't do anything with the forum...

is this due to server configuration, and some kind of server error, or due to forum and those who administer it?

Could be any one of those factors. When you say "the features are gone" do you mean they don't appear, or you get error messages when trying to use them?

This is why hourly backups rule. (www.r1soft.com)

I checked it more thoroughly, it seems that something, or someone, has deleted a table in database and created new one, empty one. therefore, all PM messages were deleted. As for the features, it is as if they are simply reset to 0.